Detection of analytes as dilute vapors requires not only a capable sensor, but also an efficient means for collecting, concentrating, and delivering the vapor analytes from the environment to the sensor. The need for the latter functionality and its challenges when the vapor is at trace levels are referred to as the “sampling problem.”
In general, the difficulties of sampling, for both aqueous and vapor sensing, stem from diffusion limits, and specifically from the time required for the vapor molecules to “find” the sensor. See, e.g., P. E. Sheehan et al., “Detection Limits for Nanoscale Biosensors,” NANO LETTERS, Vol. 5, No. 4, pp. 803-807 (2005).
These difficulties are relatively independent of sensor size. Although a larger sensor is more easily “found,” it requires more molecules to generate the same response (though larger sensors do generally benefit from a lower noise floor).
A well-known approach for enhancing sensitivity/selectivity at the cost of response time is to use a pre-concentrator that consists of a large area/volume of adsorbent material that can gather vapor molecules over time, and then with rapid heating, pump the desorbed and now concentrated vapor over the sensor. See I. Voiculescu, et al., “Microfabricated chemical preconcentrators for gas-phase microanalytical detection systems,” Trends in Analytical Chemistry, Vol. 27, No. 4, pp. 327-343 (2008). Of particular relevance to vapor sensing are W. A. Groves, et al., “Analyzing organic vapors in exhaled breath using a surface acoustic wave sensor array with preconcentration: Selection and characterization of the preconcentrator adsorbent, Anal. Chim. Acta 371, 131-143 (1998); I. Voiculescu, et al., “Micropreconcentrator for Enhanced Trace Detection of Explosives and Chemical Agents,” IEEE Sensors J. 6, 1094-1104 (2006); Q. Zhong et al., “Characterization of a high-performance portable GC with a chemiresistor array detector, Analyst 134, 283-293 (2009); M. D. Hsieh et al., “Limits of Recognition for Simple Vapor Mixtures Determined with a Microsensor Array,” Anal. Chem. 76, 1885-1895 (2004); B. Alfeeli et al., “MEMS-based multi-inlet/outlet preconcentrator coated by inkjet printing of polymer adsorbents,” Sensors and Actuators B 133, 24-32 (2008); R. E. Shaffer et al., “Multiway Analysis of Preconcentrator-Sampled Surface Acoustic Wave Chemical Sensor Array Data,” Field Anal. Chem. Tech. 2, 179-192 (1998); T. Nakamoto et al., “Odor-sensing system using preconcentrator with variable temperature,” Sensors and Actuators B 69, 58-62 (2000); and C. E. Davis et al., “Enhanced detection of m-xylene using a preconcentrator with a chemiresistor sensor,” Sensors and Actuators B 104, 207-216 (2005).
Although useful, the pre-concentrator scheme remains diffusion-limited, both in the initial collection from the ambient, and in the transfer from the pumped air stream to the sensor. For example, although it might seem that much could be gained by having a large ratio between the areas of the pre-concentrator and sensor, the bigger this ratio the faster the air stream velocity over the sensor must be and the less time there will be available for analyte to out-diffuse onto the sensor, and a fundamental diffusion limit still remains.
The key to overcoming the diffusion limit and enabling efficient collection, concentration, and delivery of analyte molecules to a sensor thus appears to involve having a way of moving the molecules by means other than a carrier gas such as air. As already noted, no artificial method, material, or apparatus currently exists for doing this and thereby for surmounting the diffusion limitation.
However, there are biological sensing systems that do achieve extraordinary levels of sensitivity and it is thought that an essential aspect is a method for molecular delivery. For example, the antennae of moths serve as means of collecting exceedingly sparse pheromone molecules from the environment (as emitted by distant females) and then delivering them (without a carrier gas) to a receptor for detection. As discussed in the next section, the invention disclosed herein provides for the first time an artificial means for accomplishing similar molecular transport, though by a mechanism different from that used biologically.